CVE-2021-20197: 
NixOS vulnerability analysis and mitigation

CVE-2021-20197 affects GNU binutils version 2.35 and earlier, specifically impacting utilities including ar, objcopy, strip, and ranlib. The vulnerability was discovered in late 2020 and publicly disclosed in March 2021. The issue involves an open race window when writing output in these utilities, which could allow an unprivileged user to gain ownership of arbitrary files through a symlink when these utilities are run as a privileged user (NVD, Sourceware Bug).

The vulnerability stems from a security flaw in the smartrename function within binutils/rename.c. The function performs chown and chmod operations on the target pathname rather than using fchown/fchmod on the file descriptor, creating a race condition. The issue has a CVSS v3.1 base score of 6.3 (Medium) with the vector CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:N. The vulnerability is classified under CWE-59 (Improper Link Resolution Before File Access) and CWE-362 (Race Condition) (NVD, [Sourceware Bug](https://sourceware.org/bugzilla/showbug.cgi?id=26945)).

When these utilities are run as a privileged user, typically as part of a script updating binaries across different users, an unprivileged user can exploit the race condition to gain ownership of arbitrary files through a symlink. In worst-case scenarios involving multiple racing file replacements, it could potentially be used to chmod arbitrary root-owned files with SUID permissions (Sourceware Bug).

The issue was fixed in GNU binutils versions after 2.35 through multiple patches that modified the smartrename function to use file descriptors instead of pathnames for permission operations. The fix includes using fchown/fchmod on file descriptors and ensuring proper handling of file states through the entire operation ([Sourceware Bug](https://sourceware.org/bugzilla/showbug.cgi?id=26945)).